JP4597978B2 - Linear repeater and optical fiber communication system - Google Patents

Description

The present invention relates to a distributed Raman amplification system that optically amplifies an optical signal in an optical fiber laid in a city as a transmission line, and a non-powered remote excitation installed away from a linear repeater or a terminal device. The present invention relates to an optical fiber communication system that performs optical amplification with a module.
This application claims priority to Japanese Patent Application No. 2004-184601 filed on June 23, 2004 and Japanese Patent Application No. 2004-292377 filed on October 5, 2004. The contents are incorporated herein.

  A configuration example of a conventional distributed Raman amplification system (DRA system) used in a wavelength division multiplexing optical fiber communication system is shown in FIGS. 21 and 22 (see, for example, Non-Patent Document 1 or 2). FIG. 21 shows the case of backward excitation DRA, and FIG. 22 shows the case of bidirectional excitation DRA. In this DRA system, a dispersion shifted fiber (DSF) is used as a transmission line, and the typical value of the zero dispersion wavelength (λ0) of the DSF is 1540 nm to 1560 nm (the specified value has a slightly wider wavelength range).

  In the case of the backward pumping DRA of FIG. 21, so-called L band 1575 to 1605 nm (typical value) is used as the wavelength of wavelength division multiplexing (WDM) signal light, and 1470 nm and 1500 nm are used as pumping light wavelengths for backward pumping. In the case of the bidirectional pumping DRA shown in FIG. 22, so-called C-band 1530 to 1560 nm (typical value) is used as the signal light wavelength, and 1420 nm and 1450 nm are used as the pumping light wavelengths for forward and backward pumping.

  The pumping light is introduced into the transmission line from the linear repeaters 1-3, 2-3, 1-4, and 2-4 using the multiplexers 14 and 24 in the opposite direction to the signal light. The excitation light source is a laser diode light source (laser diode type with a fiber Bragg grating, FBG excitation light source) having a fiber Bragg grating (FBG) as an external mirror, which is most often used.

  Each linear repeater 1-3, 2-3, 1-4, 2-4 has erbium doped fiber amplifiers (EDFA) 16, 26. The signal light that exits the linear repeaters 1-3 and 1-4 on the upstream side of the DSF and propagates through the DSF undergoes distributed Raman amplification near the linear repeaters 2-3 and 2-4 on the downstream side of the DSF. After being amplified in the transmission line in a distributed manner, it is amplified in a lumped constant by the EDFAs 16 and 26.

  In the case of FIG. 21, the SNR is improved by using the back excitation DRA. On the other hand, in the case of FIG. 22, for the purpose of further improving the SNR, a bi-directional pumping DRA in which FBG pumping light sources 13-3 and 23-3 and multiplexers 15 and 25 are added is used as the forward pumping DRA. However, in the prior art, when applying forward pumping DRA, the signal light wavelength band was limited to the C band 1530 to 1560 nm (in the present invention, as shown below, in the L band 1575 to 1605 nm, Signal light amplification by forward pumping DRA is possible).

  In the configuration of FIG. 21, when performing forward distributed Raman amplification with the aim of further improving SNR, amplification is performed using excitation light on the short wavelength side of about 100 nm of the signal light wavelength, as in the case of backward distributed Raman amplification. . The excitation light wavelengths are, for example, 1470 nm and 1500 nm (the same as in the case of backward distribution Raman amplification in FIG. 21). FIG. 23 shows the Raman gain spectrum at this time. In FIG. 23, the horizontal axis represents wavelength (nm) and the vertical axis represents gain (dB). According to FIG. 23, a flat gain spectrum is obtained in the L band 1575 to 1605 nm.

H. Masuda et al. Electron. Lett. , Vol. 35, pp. 411-412, 1999 N. Takachio et al. , OFC, PD9, pp. 1-3, 2000 M.M. D. Mermelstein et al. Electron. Lett. , Vol. 38, pp. 403-405, 2002 K. Inoue, JLT. Vol. 10, pp. 1553-1561, 1992 R. P. Espindola et al. Electron. Lett. , Vol. 38, pp. 113-115, 2002 Y. Ohki et al. , OAA, PD7, pp. 1-3, 2002 R. H. Stonen, Proc. IEEE, Vol. 68, pp. 1232-1236, 1980 H. Masuda et al. Electron. Lett. , Vol. 33, no. 12, pp. 1070-1072, 1997 H. Masuda et al. Electron. Lett. , Vol. 39, no. 23, pp. 1-2, 2003 H. Masuda et al. , IEEE Photonics Technol. Lett. , Vol. 5, no. 9, pp. 1017-1019, 1993

  FIG. 24 shows the SNR spectrum of the signal light when the forward-pumped DRA is added to the conventional DRA system of FIG. 21 as shown in FIG. In FIG. 24, the horizontal axis represents wavelength (nm) and the vertical axis represents SNR (dB). However, as described above, the signal light wavelength is L band 1575 to 1605 nm, and the excitation light wavelengths are 1470 nm and 1500 nm.

  From FIG. 24, it can be seen that significant SNR degradation occurs in the vicinity of the signal light wavelength of 1589 nm. Further, bit error rate (BER) measurement was performed as a transmission characteristic evaluation, and it was found that BER degradation occurred in a wavelength region where the SNR was about 25 dB or less. This is because the signal light wavelength (˜1589 nm) and the excitation light wavelength (1500 nm) in this wavelength range are symmetrical with respect to the zero dispersion wavelength (1545 nm), and the relative intensity noise ( RIN) transfers to signal light via stimulated Raman scattering (RIN transition), and non-degenerate four-wave mixing (ND-FWM) occurs between excitation light having a wide oscillation spectrum and signal light. (See Non-Patent Document 3 or 4).

  In the wavelength arrangement, the group speeds of the signal light and the excitation light become substantially the same, and the signal light quality deterioration due to the two phenomena (RIN transition and ND-FWM) becomes remarkable. However, the RIN transition becomes significant for an excitation light source having a large RIN such as an FBG excitation light source or a fiber laser.

  Therefore, a special low RIN excitation light source (multi-mode DFB (distributed feedback type) LD (laser diode) and iGM (inner grating multimode) LD)), which is devised as an excitation light source, is manufactured, and the DRA system (FIG. 22) However, it is reported that the RIN transition is suppressed by using forward excitation DRA (see Non-Patent Document 5 or 6). However, these special excitation light sources are expensive and have a problem that the SBS (stimulated Brillouin scattering) threshold is low. Furthermore, there is a drawback that ND-FWM cannot be suppressed. On the other hand, the FBG excitation light source and the fiber laser have a high SBS threshold.

  The present invention has been made against this background, and is the most commonly used linear repeater and forward fiber communication using forward pumping DRA that can use pumping light sources such as FBG pumping light sources and fiber lasers. The purpose is to realize the system.

The present invention relates to a silica fiber as a gain medium for Raman amplification that amplifies signal light having a plurality of wavelengths in the L band, and an excitation light source that transmits excitation light co-propagating through the silica fiber in the same direction as the signal light. , An optical fiber communication system including a multiplexer of the signal light and the excitation light installed between the silica fiber and the excitation light source , wherein the multiplexer includes a zero of the silica fiber. A signal light having a wavelength longer than the dispersion wavelength is incident, and a means for combining the signal light and the excitation light emitted from the excitation light source is provided, and the excitation light source has a longest wavelength of the excitation light. Means for emitting excitation light having a frequency difference of 13.7 to 30 THz shorter than the shortest wavelength of the signal light, the silica fiber is a dispersion-shifted fiber, and the signal light is In consideration of the nonlinear effect in the diffusion-shifted fiber, the spectrum of the signal light power is set so that the signal light power input to the silica fiber is lower at the shorter wavelength side where the Raman gain due to the Raman amplification is larger. It is characterized by being made.

  The present invention also provides a silica fiber as a Raman amplification gain medium that amplifies signal light having a plurality of wavelengths in the C band, and excitation light that transmits excitation light co-propagating through the silica fiber in the same direction as the signal light. An optical fiber communication system comprising: a light source; and a multiplexer for the signal light and the excitation light, which is installed between the silica fiber and the excitation light source, wherein the multiplexer includes the silica fiber Signal light having a wavelength longer than the zero-dispersion wavelength is incident, and the signal light and excitation light emitted from the excitation light source are combined, and the excitation light source is the longest of the excitation light. Means for emitting excitation light whose wavelength is 13.7 to 30 THz shorter than the shortest wavelength of the signal light, and the silica fiber is a non-zero dispersion shifted fiber; In consideration of the nonlinear effect in the non-zero dispersion shifted fiber, the signal light has a lower signal light power input to the silica fiber at a shorter wavelength side where the Raman gain due to the Raman amplification is larger. An optical power spectrum is set.

  The present invention also provides a silica fiber as a Raman amplification gain medium that amplifies signal light having a single wavelength, and an excitation light source that transmits excitation light that co-propagates in the silica fiber in the same direction as the signal light. An optical fiber communication system comprising a multiplexer of the signal light and the excitation light installed between the silica fiber and the excitation light source, wherein the multiplexer includes zero dispersion of the silica fiber A signal light having a wavelength longer than the wavelength is incident, and includes means for combining the signal light and the excitation light emitted from the excitation light source, and the excitation light source has a longest wavelength of the excitation light, A means for emitting excitation light having a frequency difference of 15.6 to 30 THz shorter than the wavelength of the signal light is provided.

  Alternatively, a remote excitation double-pass EDF module may be provided at the signal light output stage of the silica fiber, and the excitation light may have a wavelength of 1430 nm or more and 1470 nm or less.

  According to this, the pumping light wavelength dependency is small, and the remote pumping double-pass EDF module can be operated even with a pumping light wavelength of substantially 1430 nm.

  Alternatively, a remote excitation single-pass EDF module may be provided at the signal light output stage of the silica fiber, and the wavelength of the excitation light may be 1440 nm or more and 1470 nm or less.

  Thus, since the remote excitation single-pass EDF module has an excitation efficiency inferior to that of the double-pass EDF module, the usable pumping light wavelength is longer than that of the double-pass EDF module (eg, 1440 nm or more).

  The excitation light source may be a laser diode type with a fiber Bragg grating or a fiber laser type.

When the minimum value of the signal light wavelength is λs, the minimum value of the zero dispersion wavelength of the silica fiber is λ0, and the maximum value of the excitation light wavelength of the excitation light source is λp,
2λ0−λs> λp
The signal light wavelength, the zero dispersion wavelength, and the excitation light wavelength can be set so that Here, when the excitation light source is a laser diode type with a fiber Bragg grating having a plurality of wavelengths or a Fabry-Perot laser diode type, the signal light wavelength, the zero dispersion wavelength, and the excitation light so that 2λ0−λs> λp + 10 holds. The wavelength may be set.
Further, when the excitation light source is a fiber Raman laser type, or a laser diode type with a single wavelength fiber Bragg grating, or a laser diode type with a multiple wavelength fiber Bragg grating, or a Fabry-Perot laser diode type, The signal light wavelength, the zero dispersion wavelength, and the excitation light wavelength may be set so that 2λ0−λs> λp + 15 holds.

  As a result, this can be avoided when the worst condition that causes the maximum noise is 2λ0−λs = λp.

  At this time, the width of the plurality of wavelengths is preferably 10 nm or less.

  Furthermore, the pumping light source may include a variable attenuator for adjusting the pumping light output from each Fabry-Perot laser diode on the output side of each Fabry-Perot laser diode that performs polarization multiplexing.

  Thereby, it is possible to compensate for the difference in the excitation light wavelength (average wavelength) of each laser diode due to temperature change or manufacturing variation.

The optical fiber communication system includes an erbium-doped fiber amplifier. The erbium-doped fiber amplifier includes an erbium-doped fiber gain block including an erbium-doped fiber as a gain medium, and a front stage or a rear stage of the erbium-doped fiber gain block. A gain equalizing optical filter, an inversion distribution detection circuit for detecting an inversion distribution amount of the erbium-doped fiber, and the inversion distribution amount detected by the inversion distribution detection circuit so as to have an intended value. And an inversion distribution adjusting circuit for controlling the erbium-doped fiber gain block.
Thus, while using the erbium-doped fiber amplifier used when the forward-pumped DRA is not applied, the Raman gain spectrum newly added by the application of the forward-pumped DRA can be obtained from the erbium-doped fiber in the erbium-doped fiber gain block. Compensation can be made with the gain reduction.
The upper level occupation ratio N ₂ of the erbium-doped fiber is preferably less than 38%.

The silica fiber may be a silica fiber laid in the city. Alternatively, the silica fiber may be a concentrated optical amplification silica fiber.

  The same explanation can be made by replacing the linear repeater of the present invention with a transmitting terminal device (transmitter).

  According to the present invention, it is possible to solve the drawbacks of signal light quality degradation due to RIN transition and ND-FWM, which were problems in the prior art.

FIG. 1 is an overall configuration diagram of an optical fiber communication system according to a first embodiment.
FIG. 2 is a diagram showing the relationship between gain and wavelength in the optical fiber communication system of the first embodiment.
FIG. 3 is a diagram showing the relationship between SNR and wavelength in the optical fiber communication system of the first embodiment.
FIG. 4 is a diagram showing a wavelength relationship (in the case of DSF) in the first embodiment.
FIG. 5 is a diagram showing the relationship between signal light power and wavelength in the first embodiment.
FIG. 6 is a diagram showing the relationship between the gain and wavelength of the EDFA of the first embodiment.
FIG. 7 is an overall configuration diagram of an optical fiber communication system according to a second embodiment.
FIG. 8 is a diagram showing the wavelength relationship (in the case of NZ-DSF) of the second embodiment.
FIG. 9 is a block diagram of the main part of an optical fiber communication system according to a third embodiment.
FIG. 10 is a diagram showing the relationship between output signal light power and input pump light power in the third embodiment.
FIG. 11 is a view showing an SNR spectrum of the fourth embodiment.
FIG. 12 is a diagram showing the relationship between the LD drive current, the excitation light SNR, and the signal light SNR when an FP-LD excitation light source is used.
FIG. 13 is a diagram showing a configuration of an FP-LD excitation light source in the fourth embodiment.
FIG. 14 is a diagram showing a configuration of an EDFA installed in the linear repeater in the fifth embodiment.
[FIG. 15] A diagram showing spectra of the total stimulated emission cross section _Semi-tot and absorption cross section S _abs in the fifth embodiment.
FIG. 16 is a diagram showing changes in the gain spectrum of the EDF in the EDF gain block 53 in the fifth embodiment.
[17] In a fifth embodiment, obtained from FIG. 16, it illustrates the gain variation spectrum relative to the case upper level occupancy ratio N ₂ is 38%.
[FIG. 18] In the fifth embodiment, an example of a Raman gain spectrum when the pumping light wavelength of the forward pumping DRA is 1440 nm and a gain decrease spectrum of the EDF in the EDF gain block 53 for compensating the Raman gain spectrum are shown. FIG.
FIG. 19 is a diagram showing SNR spectra at the same Raman gain for various excitation light sources in the sixth embodiment.
FIG. 20 is a diagram showing a configuration of an optical amplifier in a seventh embodiment.
FIG. 21 is a diagram showing a conventional back excitation DRA configuration.
FIG. 22 is a diagram showing a conventional bidirectional excitation DRA configuration.
FIG. 23 is a diagram showing a Raman gain spectrum of the prior art.
[FIG. 24] A diagram showing an SNR spectrum of the prior art.

Explanation of symbols

1-1, 1-2, 1-3, 1-4, 2-1, 2-2, 2-3, 2-4 Linear repeater 10, 11, 20, 21 Silica fiber 12-1, 12-2 12-3, 13-1, 13-2, 13-3, 22-1, 22-2, 22-3, 23-1, 23-2, 23-3 FBG excitation light source 14, 15, 24, 25 Multiplexer 16, 26 EDFA
30 Remotely excited EDF module 40 Variable attenuator 41 Fabry-Perot LD
42 Polarization multiplexer 51, 53 EDF gain block 52 Gain equalizing optical filter 54 Inversion distribution detection circuit 55 Inversion distribution adjustment circuit 70 Optical amplifier

  Embodiments of the present invention will be described below with reference to the drawings. In addition, this invention is not limited to each Example demonstrated below, For example, you may combine the technical idea currently disclosed by each Example suitably.

(First Example)
FIG. 1 is an overall configuration diagram of the optical fiber communication system of the first embodiment.

  In this embodiment, as shown in FIG. 1, silica fibers 10, 11, 20, and 21 as Raman amplification gain media for amplifying signal light, and silica fibers 10, 11, 20, 21, FGB excitation light sources 12-1, 13-1, 22-1, and 23-1 that send out excitation light that co-propagates through the silica gel, silica fibers 10, 11, 20, and 21 and FGB excitation light sources 12-1 and 13- Linear repeaters 1-1, 2- having multiplexers 14, 15, 24, 25 for multiplexing the signal light and the excitation light installed between 1, 22-1 and 23-1. 1.

  Here, the feature of the present embodiment is that signal light having a wavelength longer than the zero dispersion wavelength of the silica fibers 10, 11, 20, and 21 is transmitted to the multiplexers 14, 15, 24, and 25. The FGB pumping light sources 12-1 and 13-1 are provided with means for combining the signal light and pumping light emitted from the FGB pumping light sources 12-1, 13-1, 22-1, and 23-1 , 22-1 and 23-1 are provided with means for emitting excitation light on the shorter wavelength side by 13.7 THz or more than the wavelength of the signal light.

  Silica fibers 10, 11, 20, and 21 are dispersion-shifted fibers. The signal light has a plurality of wavelengths in the L band, and the excitation light has a wavelength of 1470 nm or less.

  In the following, the first embodiment will be described in more detail.

  FIG. 1 shows an optical fiber communication system of the first embodiment. The following points are mainly different from the configuration of the prior art in FIG. That is, in this embodiment, forward distributed Raman amplification (forward DRA) is performed using excitation light of 1440 nm. The excitation light source 13-1 for the excitation light is installed in the upstream linear repeater 1-1.

  The Raman gain spectrum for this example is shown in FIG. In FIG. 2, the horizontal axis represents wavelength (nm) and the vertical axis represents gain (dB). This is a spectrum when the single excitation light wavelength of the forward DRA is changed from 1500 nm to 1440 nm toward the short wavelength side in the optical communication system of FIG. 1. Moreover, the SNR spectrum with respect to those excitation wavelengths is shown in FIG. In FIG. 3, the horizontal axis represents wavelength (nm) and the vertical axis represents gain (dB). However, the pumping light power in the case of this single pumping light wavelength was the same as the total power (300 mW) in the case of the two-wavelength (1470 nm and 1500 nm) pumping in the prior art.

  From FIG. 2, the peak of the gain spectrum occurs in the signal light wavelength range when the single pumping light wavelength is approximately 1470 nm to 1500 nm. Therefore, in the prior art, in order to obtain a high gain in the signal light wavelength region, forward DRA is performed with excitation light having a wavelength of approximately 1470 nm or more and excitation light having at least two wavelengths of excitation light having a wavelength of approximately 1500 nm or less. For example, when two-wavelength excitation light is used, the total gain spectrum is a combination of the two-wavelength gain spectra. Therefore, in the prior art, the wavelengths of the two excitation lights are selected so that the combined gain spectrum is almost flat. As described above, in the prior art, in order to obtain a high gain and a flat gain spectrum in the signal light wavelength region, it is assumed that two or more pump light wavelengths are used, and a single pump light wavelength is used, and a gain is obtained. Conventionally, it has not been considered to use an excitation light wavelength whose spectral peak is outside the signal light wavelength range. On the other hand, in this embodiment, the following single excitation wavelength is used.

  Therefore, in the prior art, the shortest signal light wavelength (1575 nm in the above example) and the longest pump light wavelength (1500 nm in the above example) are generally separated by only about 100 nm (frequency difference of about 13 THz). When the signal light wavelength is one wavelength, for example, if the signal light wavelength is 1580 nm, one wavelength is sufficient as the pumping light wavelength, but 1480 nm. This is a wavelength difference of 100 nm and a frequency difference of 12.8 THz.

  According to the SNR spectrum of FIG. 3, when the excitation light wavelengths are 1500 nm, 1490 nm, and 1480 nm, the SNR is low in the signal light wavelength range (approximately 25 dB or less). On the other hand, when the excitation light wavelengths are 1470 nm, 1460 nm, and 1440 nm, the SNR degradation in the signal light wavelength region is small, and a high SNR of about 30 dB or more is obtained. In this case (excitation light wavelengths are 1470 nm, 1460 nm, and 1440 nm), the BER characteristics were also good. Further, in this case, as shown in FIG. 2, the Raman gain in the signal light wavelength region is about 6 dB or more when the pumping light wavelength is 1470 nm, and about 4 dB or more when the pumping light wavelength is 1440 nm.

  That is, it was found that a gain large enough to secure SNR improvement by DRA was obtained although it was smaller than the gain (about 8 dB) in FIG. In addition, there is a large gain deviation in the signal light wavelength range as compared with the prior art, but this does not cause a problem by performing setting according to the wavelength of the signal light level as described later.

As described above, in the present embodiment, the longest excitation light wavelength (1470 nm in this embodiment) is greater than 100 nm (105 nm) (frequency difference of about 13.3 with respect to the shortest signal light wavelength (1575 nm). 7 THz) and set to the short wavelength side. When the signal light wavelength is one wavelength, for example, if the signal light wavelength is 1580 nm, one wavelength is sufficient as the excitation light wavelength, but it may be 1460 nm. This is a wavelength difference of 120 nm and a frequency difference of 15.6 THz.
That is, when the signal light has one wavelength, it is not necessary to ensure the flatness of the gain spectrum, and the wavelength range occupied by the signal light is generally extremely narrow compared to the case of the multi-wavelength WDM system. In the method, it can be said that a configuration having a frequency difference smaller than the above 15.6 THz is possible.

The longest excitation light wavelength may be set on the short wavelength side with a frequency difference of up to 30 THz with respect to the shortest signal light wavelength. At this time, according to Non-Patent Document 7, the Raman gain has a magnitude necessary to ensure SNR improvement by DRA. On the other hand, in other cases, that is, when the longest pump light wavelength is set to the short wavelength side with a frequency difference greater than 30 THz with respect to the shortest signal light wavelength, the Raman gain is Although it is small and there is a slight improvement in SNR due to DRA, there is not much merit as a system in view of the cost of installing an excitation light source or the like necessary for performing DRA.
For example, when the shortest signal light wavelength is 1575 nm and the frequency difference is 13.7 to 30 THz, the longest excitation light wavelength may be 1361 to 1469 nm. Incidentally, the relationship between wavelength, frequency, and speed of light is wavelength = speed of light / frequency.
Further, when the transmission path is a non-zero dispersion shifted fiber, which will be described later, when the shortest signal light wavelength is 1530 nm and the frequency difference is 13.7 to 30 THz, the longest pump light wavelength is 1327 to 1430 nm. Good.

  FIG. 4 shows the wavelength relationship regarding this embodiment (when the transmission path is a DSF). The zero dispersion wavelength is in the vicinity of 1550 nm, and the signal light wavelength region is the L band 1575 to 1605 nm. In addition, the excitation light wavelengths are 1470 nm and 1500 nm in the prior art, and are 1440 nm (1460 nm and 1470 nm may be used) in the present embodiment, for example.

  FIG. 3 shows the case where the zero dispersion wavelength is 1545 nm. For example, when the zero dispersion wavelength is 1535 nm, in addition to 1500 nm, 1490 nm, and 1480 nm, the SNR is low in the signal light wavelength region in the case of 1470 nm. (Approximately 25 dB or less).

  In this embodiment, as can be seen from the comparison between FIG. 23 and FIG. 2, the Raman gain spectrum is generally non-flat within the signal light wavelength region. In particular, the non-flatness increases as the excitation light wavelength becomes longer such as 1440 nm, 1460 nm, and 1470 nm. The relationship between signal light power and wavelength is shown in FIG. In FIG. 5, the horizontal axis represents wavelength (nm) and the vertical axis represents signal light power (dBm). Therefore, the spectrum of the signal light power input to the transmission line DSF is made non-flat according to the pumping light wavelength as shown in FIG.

  Considering the non-linear effect in the transmission line DSF, the signal light power is lowered as the wavelength increases. FIG. 6 shows the relationship between the gain and wavelength of the EDFA. In FIG. 6, the horizontal axis represents wavelength (nm) and the vertical axis represents gain (dB). Further, in order to flatten the net gain spectrum in the unit linear relay section, the gain spectrum of the EDFA is made non-flat as shown in FIG. It has been found that this can be realized by reducing the excitation level of the EDFA and reducing the average inversion distribution level (see Non-Patent Document 8) without using a gain equalization filter.

  That is, in FIG. 6, the average inversion distribution level (level-1) when the pumping light wavelength is 1440 nm is lower than the average inversion distribution level when the EDFA gain spectrum is flat in the case of the prior art, and the pumping light wavelength is The average inversion distribution level (level-2) in the case of 1460 nm may be lower than level-1.

  The above embodiment relates to a DRA system that amplifies signal light distributedly in a transmission line fiber laid on the city (land or sea floor). The length of the transmission line fiber is 40 km and 80 km, etc. is there.

  However, in consideration of the optical amplification operation of the signal light in this embodiment, the present invention can also be applied to the case where the signal light is intensively amplified like an EDFA in a linear repeater, and the gain medium is generally slightly more than in the case of DRA. It is a short (such as 10 km and 20 km) silica fiber (such as DSF). The same applies to the second and third embodiments described later.

  The excitation light source is a fiber Bragg grating (FBG) or a laser diode light source having a fiber laser as an external mirror (laser diode type with a fiber Bragg grating, FBG excitation light source or fiber laser type, fiber laser excitation light source).

  As described above, according to the present embodiment, when the transmission path is a DSF, SNR degradation that has been a problem in the prior art can be suppressed by setting a single pumping light wavelength to approximately 1470 nm or less. effective.

(Second embodiment)
FIG. 7 shows an optical fiber communication system according to the second embodiment. The configuration of the linear repeaters 1-1 and 2-1 according to the first embodiment of FIG. 1 is mainly different from the configuration of the linear repeaters 1-2 and 2-2 according to the second embodiment in the following points. That is, in this embodiment, a non-zero dispersion shifted fiber (NZ-DSF, LEAF (registered trademark), etc.) is used as a transmission line, and a typical value of zero dispersion is about 1500 nm. The signal light wavelength is C band 1530 to 1560 nm.

  The pumping light wavelengths are 1420 nm and 1450 nm in the backward pumping FBG pumping light sources 12-2 and 22-2, which are the same as those in FIG. 22 of the prior art, and the forward pumping FBG pumping light sources 13-2 and 23-2. Then, it is 1390 nm.

  The wavelength relationship in this embodiment is shown in FIG. In the prior art, as shown in FIG. 22, the pumping light wavelength of the forward DRA is 1420 nm and 1450 nm, which is the same as the pumping light wavelength of the backward DRA, but in this embodiment, as described above, the pumping light wavelength is 1390 nm. is there.

  Generally, as in the case of the first embodiment, the excitation light wavelength is set to the short wavelength side of about 13.7 THz as a frequency difference with respect to the shortest signal light wavelength (1530 nm). That is, the excitation light wavelength may be 1430 nm or less.

  Therefore, as in the case of FIG. 4, according to the present embodiment, there is an effect that SNR degradation due to RIN transition and ND-FWM, which has been a problem in the prior art, can be suppressed.

(Third embodiment)
FIG. 9 shows an optical fiber communication system of the third embodiment. The following points are mainly different from the configuration of the first embodiment of FIG. However, in FIG. 9, only points different from FIG. 1 are shown for simplicity. In the present embodiment, the remote excitation EDF module 30 is installed in the subsequent stage of the transmission path DSF (DSF-1) of the forward excitation DRA to perform remote excitation amplification. As the remote excitation EDF module 30, a double-pass type with high excitation efficiency is used (see Non-Patent Document 9).

  FIG. 10 shows the excitation characteristics of the double-pass remote excitation EDF module 30. In FIG. 10, the horizontal axis represents input pumping light power (mW), and the vertical axis represents output signal light power (dBm). With respect to the total output power of the signal light, the input pump light power dependency was measured by changing the pump light wavelength (1440, 1460, 1470, 1490 nm). From FIG. 10 and other examination results, the dependence on the pumping light wavelength is small, and the wavelength for exciting the EDF that is the gain medium provided in the remote pumping EDF module 30 is normally 1450 to 1480 nm. It has been found that the double-pass remote excitation EDF module 30 operates even at an excitation light wavelength of substantially 1430 nm.

  Further, a single-pass remote excitation EDF module can be installed instead of the double-pass remote excitation EDF module 30. In this case, the single-pass remote excitation EDF module is inferior in excitation efficiency to the double-pass remote excitation EDF module 30, so that the usable excitation light wavelength is longer than the double-pass remote excitation EDF module 30 ( 1440 nm or more).

(Fourth embodiment)
According to the first and second embodiments, the noise can be avoided in the system using the FBG pump light source or the fiber laser pump light source having a large noise caused by the ND-FWM and RIN transition, which has been a problem in the prior art. However, a typical example of the fiber laser excitation light source is a fiber Raman laser.

Here, when the minimum value of the signal light wavelength is λs, the minimum value of the zero dispersion wavelength is λ0, and the maximum value of the pumping light wavelength is λp,
2λ0−λs = λp (1)
Is the worst condition where the noise is maximized. Therefore, in the present invention, λp <1470 nm when λ0 = 1545 nm and λs = 1605 nm so as to avoid the above condition. That is, assuming that the unit of wavelength is nm,
2λ0−λs> λp + 15 (2)
It is said. However, the above values are approximate values when the excitation light source is a single wavelength FBG excitation light source.

  When the case where the excitation light source is other than the single wavelength FBG excitation light source was examined, the following was found. FIG. 11 is a diagram showing the SNR spectrum of the fourth embodiment, where the horizontal axis represents wavelength (nm) and the vertical axis represents SNR (dB). FIG. 11 shows SNR spectra at the same Raman gain for various excitation light sources.

  The types of excitation light sources are fiber Raman laser, single wavelength FBG-LD, two wavelength FBG-LD, and FP-LD (Fabry-Perot LD). The minimum value λ0 of the zero dispersion wavelength is 1530 nm. It can be seen that the SNR is low in the order of fiber Raman laser, single wavelength FBG-LD, two wavelengths FBG-LD, and FP-LD.

  At this time, the oscillation wavelength width Δλ of each excitation light source when the intensity was reduced by 10 dB was about 0.5 nm for the fiber Raman laser, about 2 nm for the single wavelength FBG-LD, and about 10 nm for the FP-LD. That is, the larger the Δλ, the higher the SNR. In the two-wavelength FBG-LD, since the number of wavelengths is twice that of the single wavelength FBG-LD, the effective Δλ is twice.

  From the above, it was found that by using an FP-LD excitation light source and a multi-wavelength FBG-LD excitation light source (abbreviated as FBG excitation light source for short), a high SNR in which the noise is suppressed can be obtained. However, when producing a multi-wavelength FBG excitation light source, it is important to narrow the wavelength interval so that the average wavelength and the maximum value λp of the excitation light wavelength are not significantly increased. However, the average wavelength is an effective value of the excitation light wavelength, and the maximum value λp is a value that determines the worst condition of the equation (1). The wavelength interval is preferably about 10 nm or less because Δλ of the single wavelength FBG-LD is about 2 nm. Note that there is no particular restriction on the lower limit of the wavelength interval (it should be larger than 0 nm), so it may be determined appropriately according to the system conditions.

  FIG. 12 shows the relationship between the LD drive current, the excitation light SNR, and the signal light SNR when the FP-LD excitation light source is used. In FIG. 12, the horizontal axis represents drive current (mA) and the vertical axis represents SNR (dB). As shown in FIG. 12, it has been found for the first time that the pumping light SNR and the signal light SNR are improved with the driving current. Therefore, there is an advantage in keeping the driving current at a high value.

  FIG. 13 shows the configuration of the FP-LD excitation light source in the fourth embodiment. In this excitation light source, the following measures are taken to obtain a high SNR. A variable attenuator 40 is installed in correspondence with each LD 41 (FP-LD) that performs polarization multiplexing to adjust the output power.

  Excitation light from each variable attenuator 40 is output after being combined by a polarization multiplexer (hereinafter referred to as PBC) 42. It was found that the excitation light wavelength (average wavelength) of each LD 41 increased with the driving current and temperature, and increased by about 3 nm per driving current of 100 mA and about 4 nm per 10 degrees of temperature.

  Further, there is a manufacturing variation of LD, which is approximately ± 5 nm. Therefore, according to the pumping light source of FIG. 13, the pumping light wavelength of each LD 41 can be adjusted by adjusting the drive current and temperature, and the pumping light power input from each LD 41 to the PBC 42 can be set to the same value by the variable attenuator 40. . Incidentally, the configuration of a normal FP-LD excitation light source is a configuration in which the variable attenuator 40 is removed from the configuration of FIG.

  For example, when the desired wavelength of the pumping light wavelength of two LDs for polarization synthesis (referred to as LD1 and LD2) is 1440 nm, the LD1 wavelength is 1444 nm and the LD2 wavelength is 1436 nm at an LD temperature of 25 ° C. due to manufacturing variations. Suppose that At this time, by setting the temperature of LD1 to 15 ° C. and the temperature of LD2 to 35 ° C., the excitation light wavelengths of LD1 and LD2 can both be set to 1440 nm (desired value). At this time, the outputs from LD1 and LD2 generally change. However, the pump light power input to the PBC 42 can be set to a desired value by the variable attenuator 40 by the individual variable attenuator 40.

(Fifth embodiment)
In this embodiment, the gain spectrum equalization method described in the first embodiment with reference to FIG. 6 (that is, a method of flattening the net gain spectrum in the unit linear relay section without using the gain equalization filter). Will be described in detail.

FIG. 14 shows a configuration example of the EDFAs 16 and 26 installed in the linear repeaters 1-1 and 1-2 in FIG. The EDFA includes an EDF gain block 51 which is a first EDF gain block disposed on the input side of signal light, an EDF gain block 53 which is a second EDF gain block disposed on the output side of signal light, A gain equalizing optical filter 52 disposed between the EDF gain block 51 and the EDF gain block 53, and an inversion of EDF (not shown) as a gain medium connected to the EDF gain block 53 and installed in the EDF gain block 53 The inversion distribution detection circuit 54 detects the distribution amount, and the inversion distribution adjustment circuit 55 is connected to the inversion distribution detection circuit 54. This inversion distribution adjustment circuit 55 excites the EDF gain block 53 so that the inversion distribution amount detected by the inversion distribution detection circuit 54 becomes a desired value by changing the pumping light power for the EDF gain block 53. Adjust the condition.

The EDF gain block 51 may not be provided, and the gain equalizing optical filter 52 may be provided at the subsequent stage of the EDF gain block 53. Further, as the inversion distribution detection circuit 55, for example, the means described in Non-Patent Document 10 may be used.

Next, the operation in this embodiment will be described. The gain (G) of the EDF in the EDF gain block 53 in dB is as follows: the proportionality constant is A, the stimulated emission cross section is _Semi (= Emission), and the upper level absorption cross section is S _ESA (= Excited State Absorption). the absorption cross section _S abs (= absorption), the stimulated emission cross section of the total _{S emi-tot = S emi +} S ESA, also the upper level occupancy ratio as _{N 2,} is given by the following equation.
_GA ( _Semi-tot N ₂ -S _abs ) (3)

FIG. 15 shows the spectrum of the total stimulated emission cross section _Semi-tot and absorption cross section S _abs , where the horizontal axis represents wavelength (nm) and the vertical axis represents the cross sectional area (normalized value). Yes. However, in FIG. 15, the peak value of these cross-sectional areas is normalized to 100. FIG. 16 shows a change in the gain spectrum of the EDF in the EDF gain block 53 obtained by using the equation (3), and the upper level occupation ratio N ₂ that is the inversion distribution amount is 42%, 40%, and 38. The change in EDF gain spectrum is shown for each of%, 36%, and 34%. In FIG. 16, the horizontal axis represents wavelength (nm) and the vertical axis represents gain (dB). As can be seen from FIG. 16, the flat gain in the L band when the upper level occupation ratio N ₂ = 38% is about 20 dB. Then, according to FIG. 16, according to the value of the upper level occupancy ratio N _2, how the gain spectrum changes can be determined quantitatively.

FIG. 17 shows the gain change amount spectrum obtained from FIG. Specifically, a gain variation spectrum based on the case where the upper level occupation ratio N ₂ is 38% (that is, when the EDF gain spectrum becomes almost flat in the signal light wavelength region) is shown, and the horizontal axis Is the wavelength (nm), and the vertical axis is the gain variation (dB). When the gain change amount is ΔG and the gain corresponding to the value of the upper level occupation ratio N ₂ is G (N ₂ ),
ΔG = G (N ₂ ) −G (N ₂ −38%) (4)
There is a relationship. As can be seen from FIG. 17, the absolute value of the gain change amount ΔG increases as the wavelength decreases in the L-band signal light wavelength region.

By using the characteristics of this gain change, the Raman gain spectrum newly added by applying the forward excitation DRA can be converted into the EDF in the EDF gain block 53 using the EDFA that is used when the forward excitation DRA is not applied. Can be compensated for by the gain decrease. Thereby, it is not necessary to provide a new EDFA in order to perform gain compensation, and economical efficiency can be ensured.

FIG. 18 shows an example of the Raman gain spectrum when the pumping light wavelength of the forward pumping DRA is 1440 nm (“added Raman gain” in the figure) and the EDF in the EDF gain block 53 that compensates the Raman gain spectrum. A gain decrease spectrum (“EDFA gain decrease” in the figure) is shown, the wavelength (nm) is taken on the horizontal axis, and the gain change (dB) is taken on the vertical axis. However, the upper level occupation ratio N ₂ of the EDF in the EDF gain block 53 is set to N ₂ = 36.5%.

Looking at the degree of coincidence of both in the signal light wavelength range of 1575 to 1605 nm, it can be seen that both agree well within 1 dB. Therefore, for example, in the configuration shown in FIG. 1, the Raman gain of the forward excitation DRA is expressed as an EDF gain block with respect to the total gain spectrum in a section (hereinafter referred to as a unit linear relay section) composed of one transmission line and one linear repeater. The spectrum can be equalized by the gain of the EDF in 53.

Further, the gain equalizing optical filter 52 can compensate for a mismatch between the Raman gain of the forward pumping DRA and the gain of the EDF in the EDF gain block 53. At this time, since the peak value of the loss spectrum of the gain equalizing optical filter 52 is small, there is also a merit that deterioration of noise characteristics of the EDFA in this embodiment can be suppressed. That is, when compensation is performed only by the gain equalizing optical filter 52 without using the technique of the present embodiment, the peak value of the loss spectrum of the gain equalizing optical filter 52 is larger than that when the technique of the present embodiment is used. Since it becomes large, the noise characteristic is deteriorated. On the other hand, if the peak value of the loss spectrum of the gain equalizing optical filter 52 is small as in this embodiment, there is a merit in terms of noise characteristics as compared with the case where compensation is performed only by the gain equalizing optical filter 52.

The set value of the upper level occupation ratio N ₂ depends on the value of the Raman gain and the proportionality constant A in Expression (3). As shown in FIG. 18, an EDF gain having a spectrum whose gain is smaller than that of the long wavelength region in the short wavelength region of the L band is obtained, and a Raman gain having a spectrum whose gain is larger than that of the long wavelength region in the short wavelength region of the L band. In order to compensate, the upper level occupation ratio N ₂ needs to be smaller than 38%. A typical value of the upper level occupation ratio N ₂ is N ₂ = 34 to 37%.

(Sixth embodiment)
The fourth embodiment shows the operating parameter values in the present invention when the transmission path is DSF. In this embodiment, operation parameter values for the case where the transmission path is NZ-DSF are shown.

In this embodiment, the typical value of the zero dispersion wavelength is about 1500 nm, and the signal light wavelength is C band 1530 to 1560 nm. However, for the present embodiment and the first to fourth embodiments, the signal light wavelength is generally a plurality of wavelengths or channels of wavelength multiplexed signals arranged over the entire band in these embodiments. In the case of the present embodiment, wavelengths or channels of wavelength multiplexed signals are arranged in the C band 1530 to 1560 nm at intervals of, for example, 100 GHz (that is, intervals of about 0.8 nm). However, in the initial stage of system operation or the like, a plurality of wavelengths of signal light are arranged in a part of the band.

Here, as described in the fourth embodiment, when the minimum value of the signal light wavelength is λs, the minimum value of the zero dispersion wavelength is λ0, and the maximum value of the pumping light wavelength is λp, 2λ0−λs = λp (Equation (1) above) is the worst condition that maximizes noise. Therefore, in this embodiment, λp <1385 nm when λ0 = 1480 nm and λs = 1560 nm so as to avoid the above condition. That is, the wavelength unit is nm, and 2λ0−λs> λp + 15 (the above formula (2)). However, the above values are approximate values when the excitation light source is a single wavelength FBG excitation light source.

When the case where the excitation light source is other than the single wavelength FBG excitation light source was examined, the following was found. FIG. 19 shows SNR spectra at the same Raman gain for various pumping light sources, with the horizontal axis representing wavelength (nm) and the vertical axis representing SNR (dB). FIG. 19 shows the case where λp = 1385 nm. The types of excitation light sources are fiber Raman laser, single wavelength FBG-LD, two wavelength FBG-LD, and FP-LD (Fabry-Perot LD). FIG. 19 shows that the SNR is low in the order of fiber Raman laser, single wavelength FBG-LD, two wavelengths FBG-LD, and FP-LD.

As SNR values, for example, at λs = 1560 nm and λp = 1385 nm, SNR = 23 dB and SNR = 28.5 dB respectively for the single wavelength FBG-LD and the two wavelengths FBG-LD. The expected minimum value of the SNR spectrum depends on system conditions such as transmission distance and transmission speed, but is typically 25 dB, for example. Therefore, in the case of the single wavelength FBG-LD, λp = 1385 nm may be sufficient. In the case of the two-wavelength FBG-LD, the wavelength of FIG. 19 can be shifted to the longer wavelength side by 5 nm, and the lowest SNR spectrum at λs = 1560 nm and λp = 1385 nm is 27 dB. Therefore, when using two-wavelength FBG-LD,
2λ0−λs> λp + 10 (5)
If it is. Further, the number of wavelengths of the FBG-LD type excitation light source may be 3 or more. Generally, the larger the number of wavelengths, the higher the SNR is obtained unless the distribution width of the excitation light wavelength is remarkably widened. The above is clearly true for the fourth embodiment.

Further, in this embodiment, the transmission path is NZ-DSF, but the spectrum of the signal light power input to NZ-DSF is made non-flat as in the case of the second embodiment. Considering the nonlinear effect in the transmission line DSF, the signal light power is lowered on the shorter wavelength side where the Raman gain is larger.

(Summary of First to Sixth Embodiments) An optical fiber communication system according to an embodiment of the present invention includes the linear repeater described in the first to sixth embodiments, and silica fibers 10, 11, 20, and 21 are transmission lines. It is realized by laying in the city as DSF.

(Seventh embodiment)
As described in the first embodiment, the DSF and NZ-DSF may be replaced with a concentrated amplification medium such as a silica fiber wound around a bobbin instead of a distributed amplification medium such as a transmission line fiber installed in the city. Good. In this case, the optical fiber communication system can be regarded as an optical amplifier, and this optical amplifier can be installed and used in a linear repeater, a transmitter and a receiver. The present embodiment relates to such an optical amplifier.

The configuration of the optical amplifier according to this example is shown in FIG. In addition, the same code | symbol is attached | subjected about the same component as what was shown in FIG. 1, and the description is abbreviate | omitted. This optical amplifier 70 has a DSF as a medium for concentrated optical amplification, the length of which is, for example, 10 km, and this DSF is wound around a bobbin. Excitation light sources used for backward Raman amplification and forward Raman amplification for DSF are FBG excitation light sources 22-1 and 13-1 having the same wavelength as in the first embodiment.

According to the present embodiment, the signal light input to the optical amplifier 70 can be amplified without RIN transition and signal quality degradation due to ND-FWM.
In addition, although the structure corresponding to 1st Example was demonstrated in the present Example, it is the same also about Examples other than 1st Example.

As mentioned above, although the Example of this invention has been described with reference to drawings, these Examples are only illustrations of this invention, and it is clear that this invention is not limited to these Examples. Accordingly, additions, omissions, substitutions, and other modifications of the components may be made without departing from the spirit and scope of the present invention.

  According to the present invention, it is possible to solve the disadvantage of signal light quality degradation caused by RIN transition and ND-FWM, which has been a problem in the prior art, so that high communication signal quality can be realized at low cost.

Claims (15)

Silica fiber as a gain medium for Raman amplification for amplifying signal light having a plurality of wavelengths in the L band ;
An excitation light source that emits excitation light that co-propagates through the silica fiber in the same direction as the signal light;
An optical fiber communication system comprising: a multiplexer of the signal light and the excitation light installed between the silica fiber and the excitation light source,
The multiplexer is provided with signal light having a wavelength longer than the zero dispersion wavelength of the silica fiber, and means for multiplexing the signal light and the excitation light emitted from the excitation light source,
The excitation light source includes means for emitting excitation light whose longest wavelength of the excitation light is 13.7 to 30 THz short wavelength side in frequency difference from the shortest wavelength of the signal light ,
The silica fiber is a dispersion shifted fiber,
In consideration of the nonlinear effect in the dispersion-shifted fiber, the signal light has a lower signal light power input to the silica fiber at a shorter wavelength side where the Raman gain due to the Raman amplification is larger. An optical fiber communication system, characterized in that an optical power spectrum is set . Silica fiber as a gain medium for Raman amplification for amplifying signal light having a plurality of wavelengths in the C band;
  An excitation light source that emits excitation light that co-propagates through the silica fiber in the same direction as the signal light;
  A combiner of the signal light and the excitation light installed between the silica fiber and the excitation light source;
  An optical fiber communication system comprising:
  The multiplexer is provided with signal light having a wavelength longer than the zero dispersion wavelength of the silica fiber, and means for multiplexing the signal light and the excitation light emitted from the excitation light source,
  The pumping light source includes means for emitting pumping light whose longest wavelength of the pumping light is 13.7 to 30 THz shorter than the shortest wavelength of the signal light.
  The silica fiber is a non-zero dispersion shifted fiber,
  In consideration of the non-linear effect in the non-zero dispersion shifted fiber, the signal light has a lower signal light power input to the silica fiber at a shorter wavelength side where the Raman gain due to the Raman amplification is larger. The spectrum of the signal light power is set
  An optical fiber communication system. Silica fiber as a gain medium of Raman amplification that amplifies signal light of a single wavelength;
  An excitation light source that emits excitation light that co-propagates through the silica fiber in the same direction as the signal light;
  A combiner of the signal light and the excitation light installed between the silica fiber and the excitation light source;
  An optical fiber communication system comprising:
  The multiplexer is provided with signal light having a wavelength longer than the zero dispersion wavelength of the silica fiber, and means for multiplexing the signal light and the excitation light emitted from the excitation light source,
  The excitation light source includes means for emitting excitation light whose longest wavelength of the excitation light is 15.6 to 30 THz shorter than the wavelength of the signal light.
  An optical fiber communication system. A remote excitation double-pass EDF module is provided at the signal light output stage of the silica fiber,
The optical fiber communication system according to any one of claims 1 to 3, wherein the excitation light has a wavelength of 1430 nm or more and 1470 nm or less. A remote excitation single-pass EDF module is provided at the signal light output stage of the silica fiber,
The optical fiber communication system according to any one of claims 1 to 3, wherein the excitation light has a wavelength of 1440 nm or more and 1470 nm or less. 6. The optical fiber communication system according to claim 1, wherein the excitation light source is a laser diode type with a fiber Bragg grating or a fiber laser type. When the minimum value of the signal light wavelength is λs, the minimum value of the zero dispersion wavelength of the silica fiber is λ0, and the maximum value of the excitation light wavelength of the excitation light source is λp,
2λ0−λs> λp
Optical fiber communication system according to claim 1 or 2, the signal light wavelength, as established setting the zero dispersion wavelength, and the excitation light wavelength. The excitation light source is a laser diode type with a fiber Bragg grating of multiple wavelengths or a Fabry-Perot laser diode type,
2λ0−λs> λp + 10
The optical fiber communication system according to claim 7, wherein the signal light wavelength, the zero dispersion wavelength, and the pumping light wavelength are set so that: The excitation light source is a fiber Raman laser type, a laser diode type with a single wavelength fiber Bragg grating, a laser diode type with a multiple wavelength fiber Bragg grating, or a Fabry-Perot laser diode type,
2λ0−λs> λp + 15
The optical fiber communication system according to claim 7, wherein the signal light wavelength, the zero dispersion wavelength, and the pumping light wavelength are set so that The optical fiber communication system according to claim 8 or 9, wherein a width of the plurality of wavelengths is 10 nm or less. 10. The optical fiber communication system according to claim 8 or 9, wherein the pumping light source includes a variable attenuator for adjusting pumping light output from each Fabry-Perot laser diode on the output side of each Fabry-Perot laser diode that performs polarization multiplexing. . The optical fiber communication system has an erbium-doped fiber amplifier,
The erbium-doped fiber amplifier is
An erbium-doped fiber gain block with an erbium-doped fiber as a gain medium;
A gain equalizing optical filter installed before or after the erbium-doped fiber gain block;
An inversion distribution detection circuit for detecting an inversion distribution amount of the erbium-doped fiber;
Optical fiber communication system according to claim 1 or 2 wherein said inversion amount and an inversion adjustment circuit that controls the erbium doped fiber gain block such that the desired value to be detected by the inversion detection circuit. The optical fiber communication system according to claim 12, wherein an upper level occupation ratio N2 of the erbium-doped fiber is less than 38%. 4. The optical fiber communication system according to claim 1, wherein the silica fiber is a silica fiber laid in a city. The optical fiber communication system according to any one of claims 1 to 3, wherein the silica fiber is a silica fiber for concentrated light amplification.

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